1. Technical Field
The present disclosure relates to electrosurgical devices suitable for use in surface ablation applications and, more particularly, to electromagnetic energy delivery devices including an energy applicator array and electrosurgical systems including the same.
2. Discussion of Related Art
Treatment of certain diseases requires the destruction of malignant tissue growths, e.g., tumors. Electromagnetic radiation can be used to heat and destroy tumor cells. Treatment may involve inserting ablation probes into tissues where cancerous tumors have been identified. Once the probes are positioned, electromagnetic energy is passed through the probes into surrounding tissue.
In the treatment of diseases such as cancer, certain types of tumor cells have been found to denature at elevated temperatures that are slightly lower than temperatures normally injurious to healthy cells. Known treatment methods, such as hyperthermia therapy, heat diseased cells to temperatures above 41° C. while maintaining adjacent healthy cells below the temperature at which irreversible cell destruction occurs. These methods involve applying electromagnetic radiation to heat, ablate and/or coagulate tissue. Microwave energy is sometimes utilized to perform these methods. Other procedures utilizing electromagnetic radiation to heat tissue also include coagulation, cutting and/or ablation of tissue.
Electrosurgical devices utilizing electromagnetic radiation have been developed for a variety of uses and applications. A number of devices are available that can be used to provide high bursts of energy for short periods of time to achieve cutting and coagulative effects on various tissues. There are a number of different types of apparatus that can be used to perform ablation procedures. Typically, microwave apparatus for use in ablation procedures include a microwave generator that functions as an energy source, and a microwave surgical instrument (e.g., microwave ablation probe) having an antenna assembly for directing the energy to the target tissue. The microwave generator and surgical instrument are typically operatively coupled by a cable assembly having a plurality of conductors for transmitting microwave energy from the generator to the instrument, and for communicating control, feedback and identification signals between the instrument and the generator.
There are several types of microwave probes in use, e.g., monopole, dipole and helical, which may be used in tissue ablation applications. In monopole and dipole antenna assemblies, microwave energy generally radiates perpendicularly away from the axis of the conductor. Monopole antenna assemblies typically include a single, elongated conductor. A typical dipole antenna assembly includes two elongated conductors that are linearly aligned and positioned end-to-end relative to one another with an electrical insulator placed therebetween. Helical antenna assemblies include helically-shaped conductor configurations of various dimensions, e.g., diameter and length. The main modes of operation of a helical antenna assembly are normal mode (broadside), in which the field radiated by the helix is maximum in a perpendicular plane to the helix axis, and axial mode (end fire), in which maximum radiation is along the helix axis.
A microwave transmission line typically includes a long, thin inner conductor that extends along the longitudinal axis of the transmission line and is surrounded by a dielectric material and is further surrounded by an outer conductor around the dielectric material such that the outer conductor also extends along the transmission line axis. In one variation of an antenna, a waveguiding structure, such as a length of transmission line or coaxial cable, is provided with a plurality of openings through which energy “leaks” or radiates away from the guiding structure. This type of construction is typically referred to as a “leaky coaxial” or “leaky wave” antenna. The design of the microwave applicator radiating antenna(s) influences the thermal distribution.
Electric power is generally measured in watts (W), or joules per second. The electromagnetic-energy absorption rate in biological tissue, sometimes referred to as the specific absorption rate (SAR), indicates the energy per mass unit absorbed in the tissue and is usually expressed in units of watts per kilogram (W/kg), and may be expressed as
                                          S            ⁢                                                  ⁢            A            ⁢                                                  ⁢            R                    =                                    1              2                        ⁢                          σ              ρ                        ⁢                                                          E                                            2                                      ,                            (        1        )            where σ is the tissue electrical conductivity in units of Siemens per meter (S/m), ρ is the tissue density in units of kilograms per cubic meter (kg/m3), and |E| is the magnitude of the local electric field in units of volts per meter (V/m).
The relationship between the initial temperature rise ΔT (° C.) in tissue and the specific absorption rate may be expressed as
                                          Δ            ⁢                                                  ⁢            T                    =                                    1              c                        ⁢            S            ⁢                                                  ⁢            A            ⁢                                                  ⁢            R            ⁢                                                  ⁢            Δ            ⁢                                                  ⁢            t                          ,                            (        2        )            where c is the specific heat of the tissue (in units of Joules/kg-° C.), and Δt is the time period of exposure in seconds (sec). Substituting equation (1) into equation (2) yields a relation between the induced temperature rise in tissue and the applied electric field as
                              Δ          ⁢                                          ⁢          T                =                              1            2                    ⁢                      σ                          ρ              ⁢                                                          ⁢              c                                ⁢                                                  E                                      2                    ⁢          Δ          ⁢                                          ⁢                      t            .                                              (        3        )            
As can be seen from the above equations, modifying the local electric-field amplitude directly affects the local energy absorption and induced temperature rise in tissue. In treatment methods such as hyperthermia therapy, it would be desirable to deposit an electric field of sufficient magnitude to heat malignant tissue to temperatures above 41° C. while limiting the SAR magnitude in nearby healthy tissue to be less than that within the tumor to keep the healthy cells below the temperature causing cell death. In existing, multiple, microwave applicator systems for hyperthermia treatment, the overall heating pattern produced by the multiple applicators may be a combination of the individual heating patterns produced by each separate applicator, or a result of the super-position of electromagnetic waves from all the applicators in the system.
Unfortunately, during certain procedures, clinicians cannot accurately predetermine or manually adjust the settings for output power and phase of multiple microwave applicators to focus heat reliably, making it difficult to determine the area or volume of tissue that will be ablated.